1. Field of the Invention
This invention relates to a process for producing phosphoric acid and dicalcium phosphate. More particularly, the invention provides a process for obtaining relatively pure phosphoric acid and/or impure fertilizer-grade dicalcium phosphate from low-grade phosphatic raw materials as well as from high-grade phosphate rock.
2. Description of the Prior Art
The manufacture of phosphoric acid is conventionally performed on a commercial scale by either the wet process or the thermal process. In the wet process, phosphate rock is treated with sulfuric acid to produce weak aqueous phosphoric acid (20 - 30% P.sub.2 O.sub.5) and calcium sulfate. The phosphoric acid and calcium sulfate are separated by filtration and the phosphoric acid is concentrated to the desired level (usually 40 to 72% P.sub.2 O.sub.5). The resulting acid is quite impure and usually black in color. It is suitable primarily for fertilizer manufacture and not for other industrial purposes.
Accordingly, the present commercial wet process for producing phosphoric acid employs the highest grade of phosphate rock obtainable since this results in the lowest impurities content of the resulting acid. However, high grade phosphate rock is becoming very scarce and expensive. In Florida, for example, there is very little phosphate rock available containing as high as 35% P.sub.2 O.sub.5 but there are large quantities of lower grade phosphate rock containing around 30% P.sub.2 O.sub.5 and lower. However, use of these lower grade phosphate rocks results in a very impure phosphoric acid which is undesirable for most uses. Also, there are available large quantities of rejected materials from phosphate rock beneficiation such as phosphate slimes containing 12 to 14% P.sub.2 O.sub.5 and colloidal phosphate containing around 18% P.sub.2 O.sub.5.
The thermal process consists of the reduction of phosphate rock to elemental phosphorus and the conversion of the elemental phosphorus by oxidation to P.sub.2 O.sub.5. However, this process is expensive and therefore it is not widely used for fertilizer manufacture.
In addition to obtaining phosphoric acid, various phosphates which are useful in phosphate fertilizers, such as monocalcium phosphate, may also be obtained from phosphate rock by various processes. For example, Ross, et al, U.S. Pat. No. 1,191,615 shows a wet process for producing a concentrated fertilizer by which monocalcium phosphate is obtained from phosphate rock by utilizing sulfuric and phosphoric acids.
Monocalcium phosphate may also be obtained from phosphate rock by subjecting the ground raw material to sulfur dioxide in the presence of water as disclosed in the patent to Blumenberg, U.S. Pat. No. 1,251,741.
Another method of obtaining monocalcium phosphate from phosphate rock is disclosed in U.S. Pat. No. 2,914,380 to Vicheng. According to this process, crude phosphate material is treated with phosphoric acid to produce a water soluble product which is readily separated from any insoluble residual material. Pure monocalcium phosphate may then be obtained by extracting the phosphoric acid from this solution with an organic solvent, such as butanol. When the phosphoric acid concentration in the aqueous phase has been sufficiently diminished, the monocalcium phosphate precipitates.
It is also known that monocalcium phosphate undergoes a dissociation reaction to form phosphoric acid and dicalcium phosphate as follows: EQU Ca(H.sub.2 PO.sub.4).sub.2 .fwdarw. H.sub.3 PO.sub.4 + CaHPO.sub.4 ( 3)
the extent to which this reaction goes to the right depends upon various conditions. K. L. Elmore and T. D. Farr, reported in Industrial and Engineering Chemistry, Volume 32, pages 580 - 6, April 1940, that in aqueous systems high temperatures and high concentrations of monocalcium phosphate favor high conversion of monocalcium phosphate to phosphoric acid and dicalcium phosphate. It was also reported by Andre Boulle and Armelle de Sallier Dupin in Compt. rend. Vol. 248, pages 1669-72 (1959) that the above reaction takes place to a substantial extent in the presence of many organic compounds, including ethanol, acetone, dioxane, tetrahydrofuran and pyridine.